In the continuing effort to produce smaller electronic devices, manufacturers of such devices are, in some circumstances, dispensing with the packaging material which forms a large part of the volume of a typical chip. An example of such technology is the so called "flip chip" device. Balls of solder are applied directly to the underside of the sections of wafer destined to be electronic chips, which sections are known as dies. These solder balls allow direct connection of each flip chip after the wafer is cut into individual devices. The facilitation of external connections is thus moved from the back end (packaging) to the front end (wafer fabrication) of a microelectronic device production line. Where high volume, high efficiency production is required, as in the manufacture of flip chip ball grid array (FC-BGA) devices, a need exists for rapid and accurate measurement of the solder balls or bumps.
Systems are known for separate two dimensional (2D) and three dimensional (3D) imaging of object features. A two dimensional imaging system can provide information regarding whether or not a part of an object covers a certain X-Y point in space. For example, on a flip chip device a 2D imaging system would determine the maximum extent of the hemispherically shaped volume of solder comprising a ball as projected in the X-Y plane. Thus, a 2D system would obtain data indicating that a circular shaped region of the X-Y plane was covered by the ball. Such a 2D system collects no information regarding the height, or Z dimension, of the ball. Such 2D measuring systems generally use charged coupled devices (CCDs) and can rapidly and simultaneously collect 2D data for a selected region of a target object. Such 2D systems have the advantage of high speed collection but obviously lack information on the Z dimension.
A 3D imaging system can collect X, Y and Z data regarding dimensions of a target object. For the example of a single ball of a flip chip device, a 3D system would obtain data containing the height of the ball at each X-Y point or equivalently, the X-Y extent of the ball at each height Z. Such 3D data is generally collected by directing one or more laser beams over the surface of the target object with data for each laser beam collected sequentially by a CCD or position sensitive device (PSD). Such devices usually rely on optical triangulation techniques. While the systems provide full 3D data, they are much slower than 2D systems due to the volume of the extra data and the sequential method of collection.
Three dimensional optical sensor systems utilizing laser optical triangulation have been used to inspect chip packages. Such a system typically consists of a semiconductor diode laser, a beam deflector (for example, an acousto-optical (AO) deflector, also called an AO modulator) and a position sensitive device (PSD). The laser diode provides the light source for measurements. The beam deflector directs the laser beam to sweep the directed light over a narrow strip of area on the target object. To cover the entire target area, such systems typically rely on a mechanical translation, of the target object, or in some circumstances, the sensor system, in a direction perpendicular to the AO sweep direction. The PSD measures the height of the target object at each scan point and the data are stored until an image record for the entire object or a selected portion is collected. The stored image record may then be compared to a manufacturer's specification for the object or a selected portion thereof to determine whether the object or portion of interest meets specification.
Note that while systems such as those described herein are often generally referred to as "scanners" or "scanning systems", as used herein, the terms "scan" and "scanning" refer to the steps of directing light upon a target object and collecting light reflected from the target object into a photodetector to generate one or more electrical signals that are stored in a memory device, or buffer. This definition provides a distinction from the subsequent processing steps applied to these stored raw data to generate an image or to perform any other analysis required such as comparing the data to manufacturer specifications.
As used herein, the term "image" (a noun) refers to any collection of information in tangible form representing properties of an object where the data is categorized according to geometrical location. The term "image" (a verb), as used herein, may have two meanings depending on the context. It may refer to the overall process of scanning an object to obtain raw data followed by the processing of that data to produce an image. For example, "the object was imaged". Where the scanning step is distinguished from the overall process of producing an image of an object, the term "image" may refer only to the post-scanning processing of the raw data. For example, "the object was scanned and imaged". Accordingly, appreciation of the separability of the scanning and imaging phases of acquiring a final image of a target object is important to understanding the present invention.
U.S. Pat. No. 5,554,858 issued to Costa et al. ("Costa '858"), expressly incorporated herein by reference, describes a 3D imaging system. A laser light source combined with an AO deflector is positioned to illuminate an object and sweep along the AO deflection direction while commercially available linear motion tables provide the transverse translation. PSD sensors are positioned on both sides of the incident beam to receive light reflected from the sample and focused into the PSDs by lenses. Further, Costa '858 describes use of multi-channel PSDs to collect the imaging data. A PSD provides an analog output current ratio proportional to the position of a light spot falling along its length. A multi-channel PSD has a segmented photo-sensitive area, the multiple segments comprising the multiple data channels. When used with a selective sampling technique, the multi-channel PSD can reduce the effect of stray or multiply reflected light.
U.S. Pat. No. 5,859,924 to Liu et al. ("Liu '924"), expressly incorporated herein by reference, describes another such system. Liu '924 describes another imaging system utilizing optical triangulation techniques. This system uses a laser beam and AO deflector and one or more photosensitive devices to collect light reflected off-axially (from the axis of the incident light source) from the target object. Further, the system uses a polarizing beam splitter in the optical path of the incident beam to direct light reflected co-axially from the target object into a photo diode array for intensity measurement.
U.S. patent application Ser. No. 09/095,367 ("Liu App. '367"), filed Jun. 6, 1998, expressly incorporated herein by reference, describes a 3D scanning system in which multiple laser beams are swept across a sample by a beam deflector and the multiple reflected beams are imaged into one or more PSDs or CCDs for determination of a 3D profile of the target object.
U.S. patent application Ser. No. 09/019,479 ("Liu App. '479"), filed Feb. 5, 1998, expressly incorporated herein by reference, describes a system and method for selective scanning of an object or pattern including scan correction.
While the speed of 3D scanning has been improving, 3D scanning has still remained a process limiting step. It has therefore been desired to find ways to further increase the speed of acquiring desired 3D data about target objects.